The worlds first cache that crosses over to a virtual world???
That’s correct folks there is a geocache in Second Life Virtual Geocache (GA0483) on Geocaching Australia. 
So how about ‘in world’ geocaching?
Second Life residents can travel to Houlihan, pick up their in-world GPS unit and locate their first cache. The GPS unit is a held item that beeps with increased frequency the closer to the cache. They can then log into the geocaching website and log their find. 
Second Life : Geocaching in SL
This will need further investigation 😉